1. Field of the Invention
The present invention relates to an apparatus and method for transmitting a signal based on interleaving delay diversity; and, more particularly, to an apparatus and method for transmitting a signal based on interleaving delay diversity which supports high data rate transmission by using a multi-code transmission system.
2. Description of Related Art
While existing mobile communication systems provide voice-based service, a third-generation mobile communication system, which is under active development, supports multimedia service. To provide the multimedia service, the capacity of the mobile communication system should be expanded greatly. Therefore, a diversity technology that can expand system capacity without additional bandwidth increase is studied actively.
Diversity includes space diversity, angle diversity, polarization diversity, field diversity, frequency diversity, multi-path diversity and time diversity. The space diversity can be acquired by using a plurality of transmission antennas or a plurality of reception antennas. The space between the antennas should be enough for each diversity branch to have uncorrelated fading.
This is hard to be achieved when the size of the receiver is small. Also, the signals can hardly be received at the same intensity, and the cost and power supply for a mobile terminal are problematic, too. Therefore, it is desirable and economical to apply the space diversity to a base station with less complexity and sufficient space for installing a plurality of antennas.
The angle diversity can be acquired by selecting a plane wave so that the directional antennas should have uncorrelated fading. The field diversity can be acquired by using a certain point in which elements of electric field and magnetic field are uncorrelated. The frequency diversity uses a plurality of channels that are apart more than a coherent bandwidth from each other. The multi-path diversity can be acquired by separating signals transmitted through a rake receiver with different time delay. The time diversity can be acquired by sending out identical signals at several time slots.
FIG. 1 is a block diagram illustrating a conventional multi-code transmitter. The multi-code transmitter includes a converting unit 110, a modulation unit 120, an adder 130, and a multiplication unit 140. The converting unit 110 converts an input data bit having a bit length of Tb in parallel into a plurality of sub-channels of low data rate bit streams. The modulation unit 120 modulates the low data rate bit streams from the converting unit 110 by using a Walsh code according to each sub-channel. The adder 130 receives the modulated data from the modulation unit 120 and adds the modulated data. The multiplication unit 140 applies a pseudo-noise (PN) code to an output signal of the adder 130.
Hereafter, the operation of a conventional multi-code transmitter will be described. The bit streams from the converting unit 110 have a symbol length of T=KTb. Due to the increase in the symbol length in each sub-channel, the system performance becomes less sensitive to multi-path delay spread.
Subsequently, sub-channels are obtained by modulation in the modulation unit 120 and discriminated. By doing so, multi-path interference can be decreased. Also, processing gain (N) is adjusted properly so that the bandwidth required for data stream after modulation could be the same as the bandwidth of the original high rate data stream. A binary data signal and a Walsh code in a k_th sub-channel are expressed as follows:
                                          b            k                    ⁡                      (            t            )                          =                              ∑                          i              =              ∞                        ∞                    ⁢                                          ⁢                                    b              i              k                        ⁢                                          p                t                            ⁡                              (                                                                            (                                              i                        -                        1                                            )                                        ⁢                    T                                    ,                                      i                    ⁢                                                                                  ⁢                    T                                                  )                                                                        Equation        ⁢                                  ⁢        1                                                      W            k                    ⁡                      (            t            )                          =                              ∑                          i              =              1                        N                    ⁢                                          ⁢                                    W              i              k                        ⁢                                          p                t                            ⁡                              (                                                                            (                                              i                        -                        1                                            )                                        ⁢                                          T                      c                                                        ,                                      i                    ⁢                                                                                  ⁢                                          T                      c                                                                      )                                                                        Equation        ⁢                                  ⁢        2            
T denotes a symbol length; Pt(t1,t2) denotes a unit rectangular pulse in a time slot [t1,t2]; and Tc denotes a chip length.
Also, bik and/or Wik is 1 or −1 and/or their probability for having each value is the same, i.e., ½. The PN code can be expressed as Equation 3:
                              PN          ⁡                      (            t            )                          =                              ∑                          i              =              1                        K                    ⁢                                          ⁢                                    PN              i                        ⁢                                          p                t                            ⁡                              (                                                                            (                                              i                        -                        1                                            )                                        ⁢                                          T                      c                                                        ,                                      i                    ⁢                                                                                  ⁢                                          T                      c                                                                      )                                                                        Equation        ⁢                                  ⁢        3            
PNi is 1 or −1 and PNi equals to PNi+N with respect to all i values (PNi=PNi+N). The chip length Tc has a value of T/N. A transmission signal of the mobile communication system can be expressed as Equation 4:
                              S          ⁡                      (            t            )                          =                              ∑                          k              =              1                        K                    ⁢                                          ⁢                                                    2                ⁢                P                                      ⁢                                          b                k                            ⁡                              (                t                )                                      ⁢                                          W                k                            ⁡                              (                t                )                                      ⁢                          PN              ⁡                              (                t                )                                      ⁢                          cos              ⁡                              (                                                      ω                    c                                    ,                  t                                )                                                                        Equation        ⁢                                  ⁢        4            
P denotes power consumed for transmitting a signal.
When a channel through which the transmission signal passes is
            h      ⁡              (        t        )              =                  ∑                  l          =          1                L            ⁢                          ⁢              β        ⁢                                  ⁢                  δ          ⁡                      (                          t              -                              τ                l                                      )                          ⁢                  ⅇ                      j            ⁢                                                  ⁢                          γ              l                                            ,a signal received in a receiver is expressed as Equation 5:
                                                                        y                ⁡                                  (                  t                  )                                            =                                                ∑                                      k                    =                    1                                    K                                ⁢                                                                  ⁢                                                      ∑                                          l                      =                      1                                        L                                    ⁢                                                                          ⁢                                                                                    2                        ⁢                        P                                                              ⁢                                          β                      l                                        ⁢                                                                  b                        k                                            ⁡                                              (                                                  t                          -                          τ                                                )                                                              ⁢                                                                  W                        k                                            ⁡                                              (                                                  t                          -                          τ                                                )                                                                                                                                                                                                                ⁢                                                                    PN                    ⁡                                          (                                              t                        -                        τ                                            )                                                        ⁢                                      cos                    ⁡                                          (                                                                                                    ω                            c                                                    ⁢                          t                                                +                                                  γ                          l                                                                    )                                                                      +                                  n                  ⁡                                      (                    t                    )                                                                                                          Equation        ⁢                                  ⁢        5            
βl denotes a channel coefficient; τ denotes a channel time delay; γl denotes a phase shift; L denotes the number of multi-paths; and n(t) denotes white Gaussian noise having double sided power spectral density.
When it is assumed that the time delay and phase of a carrier can be restored completely in the receiver, an output of a correlator of the receiver can be expressed as Equation 6.Zi=∫iT+τy(t)Wk(t−τi)PN(t−τi)cos (ωct+ri)dt  Equation 6
However, the conventional multi-code transmitter, which is described above, requires additional parallel codes as data rate is increased, thus increasing the amount of data processing and complexity.